A radio frequency (RF) filter is a device that is utilized to allow or stop selected RF signals in a specific range of frequencies, or used to eliminate/filter out any unwanted RF signals. That is, an RF filter is designed to allow for attenuation or transmission of a range of frequencies that would be applied. For example, an RF filter in a wireless device is used to receive designated RF signals and also helps to cut RF interference that could occur if a hairdryer, lamp, or other “noisy” device is activated. Four general filter functions are desirable: a band-pass filter that selects only a desired band of frequencies, a band-stop filter that eliminates an undesired band of frequencies, a low-pass filter that allows only frequencies below a cutoff frequency to pass, and a high-pass filter that allows only frequencies above a cutoff frequency to pass.
Radio frequency (RF) and microwave filters usually are designed to operate on signals in the megahertz (MHz) to gigahertz frequency (GHz) ranges (medium frequency to extremely high frequency). This frequency range is the range used by most broadcast radio, television, wireless communication (cell phones, Wi-Fi, etc. . . . ), and thus most RF and microwave devices will include some kind of filtering on the signals transmitted or received. Such filters are commonly used as building blocks for duplexers to combine or separate multiple frequency bands. In general, RF and microwave filters are most commonly made up of one or more coupled resonators, and thus any technology that can be used to make resonators can also be used to make filters.
Currently, radio frequency (RF) filters for receiving and transmitting radio waves in the selected frequency band utilizes several known technologies. For example, coaxial filter uses coaxial transmission lines providing higher quality factor than planar transmission lines, and is thus used when higher performance is required. The coaxial resonators may make use of high-dielectric constant materials to reduce their overall size. However, the dimension of a resonator filter is constrained by the pass band frequency and its physical size cannot be reduced as desired.
The most commonly used high power radio frequency (RF) filter is cavity filter. Cavity filter (e.g. waveguide filter) offers high quality factor (Q factor), which indicates a lower rate of energy loss. Well constructed cavity filters are capable of high selectivity even under power loads of at least a megawatt. Higher Q quality factor, as well as increased performance stability at closely spaced (down to 75 kHz) frequencies, can be achieved by increasing the internal volume of the filter cavities. Physical length of conventional cavity filters can vary from over 82″ in the 40 MHz range, down to under 11″ in the 900 MHz range. In the microwave range (1000 MHz (or 1 GHz) and higher), cavity filters become more practical in terms of size and a significantly higher quality factor than lumped element resonators and filters, though power handling capability may diminish. Similar to coaxial resonator filter, however, the dimension of a cavity filter is also determined by the pass band frequency. Therefore, its physical size cannot be reduced.
Pucks made of various dielectric materials can be used as an alternative to make resonators for dielectric filters. As with the coaxial resonators, high-dielectric constant materials may be used to reduce the overall size of the filter. With low-loss dielectric materials, these can offer significantly higher performance than the other technologies previously discussed. Electro-acoustic resonators based on piezoelectric materials can be used for filters. Since acoustic wavelength at a given frequency is several orders of magnitude shorter than the electrical wavelength, electro-acoustic resonators are generally smaller than electromagnetic counterparts such as cavity resonators. A common example of an electro-acoustic resonator is the quartz resonator which essentially is a cut of a piezoelectric quartz crystal clamped by a pair of electrodes. This technology is limited to some tens of megahertz. For microwave frequencies, thin film technologies such as surface acoustic wave (SAW) and, bulk acoustic wave (BAW) have been used for filters. Although dielectric resonator filter offers superior properties, the production of dielectric resonator filters depends on rare earth materials. Thus the cost is high, and dimensions are still too big.
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C. When connected together, they can act as an electrical resonator, an electrical analogue of a tuning fork, storing electrical energy oscillating at the circuit's resonant frequency. In a LC circuit, the pass band frequency is determined by the resonant frequency. The relation between resonant frequency (f0 in Hertz) and the values of LC and C is described as
      f    0    =            1              2        ⁢        π        ⁢                              L            ⁢                                                  ⁢            C                                .  LC circuit is a classical RF filter. However, due to current limitations of the L and the C devices, it cannot be used in high quality factor and high power handling applications such as base stations.
Striplines, which is supported by dielectric substrate on both sides, have also been used in RF filter applications. However, such filter cannot handle high RF power. Furthermore, the quality factor (Q value) of this type of filter is limited due to the additional substrate loss.